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Why microbes are smarter than you thought

By Michael Marshall

The vast majority of species on Earth are single-celled. Most of these languish in obscurity – many have never even been named – but some of the relatively few species that have been studied exhibit remarkable abilities.

But many bacteria and protists also exhibit behaviour that looks remarkably intelligent. This behaviour isn’t the result of conscious thought – the sort you find in humans and other complex animals – because single-celled organisms don’t have nervous systems, let alone brains.

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Communication

Bacteria talk to each other with chemicals. They do so for a host of reasons, some of them hard to understand unless you are another bacterium (or a dedicated bacteriologist), but one of the most straightforward is demonstrated by Bacillus subtilis.

If B. subtilis individuals are growing in a food-poor area, they release chemicals into their surroundings. These essentially tell their neighbours&colon; “There’s not much food here, so clear off or we’ll both starve.”

In response to these chemical messages, the other bacteria set themselves up further away, completely changing the shape of the colony.

Decision-making

Many single-celled organisms can work out how many other bacteria of their own species, are in their vicinity – an ability known as “quorum sensing”.

Each individual bacterium releases a small amount of a chemical into the surrounding area – a chemical that it can detect through receptors on its outer wall. If there are lots of other bacteria around, all releasing the same chemical, levels can reach a critical point and trigger a change in behaviour.

Pathogenic (disease-causing) bacteria often use quorum sensing to decide when to launch an attack on their host. Once they have amassed in sufficient numbers to overwhelm the immune system, they collectively launch an assault on the body. Jamming their signals might provide us with a way to fight back.

City living

Not only can bacteria be talkative and co-operative, but they also form communities. When they do, the result is a biofilm, most familiar as the thin layers of slime that coat the insides of water pipes, or kitchen surfaces in student residences. They’re also found in biological refuges, like the inner linings of human digestive systems – anywhere, in fact, where there is plenty of water.

Many different species live side by side in these “bacterial cities”, munching one another’s wastes, cooperating to exploit food sources, and safeguarding one another from external threats – such as antibiotics.

Accelerated mutation

Many microbes can accelerate the rate at which their genes mutate. This allows them to obtain new abilities that may be helpful when conditions get tough. This is a risky strategy, since many of the new mutations will be harmful or even fatal and is, in effect, a last-ditch tactic when there’s little left to lose.

Navigation

It’s common knowledge that many animals can navigate across vast distances, migrating birds and honeybees being among the best-known examples. But microbes are also pretty good at it.

The single-celled algae collectively called Chlamydomonasswim towards light, but only if it is of a wavelength that they can use for photosynthesis.

Similarly, some bacteria move according to the presence of chemicals in their environment – a behaviour called chemotaxis. E. coli, for example, move like sharks on the trail of blood if a few molecules of food are dropped into their environment.

Another group of bacteria align themselves to the Earth’s magnetic field, allowing them to head directly north or south (Science, DOI&colon; 10.1126/science.170679). Known as magnetotactic bacteria, their special ability comes from specialised organelles loaded with magnetic crystals.

But perhaps the most striking feat of microbial navigation is performed by the slime mould Physarum polycephalum. This colony of amoeba-like organisms always finds the shortest route through a maze.

E. coli goes one better. This bacterium spends part of its life cycle travelling through the human digestive system encountering different environments as it goes. In the course of its journey, it encounters the sugar lactose before it finds the related sugar, maltose. At its first taste of lactose, it switches on the biochemical machinery to digest it – but it also partially activates the machinery for maltose, so that it will be ready for a feast as soon as it is reached.

To show that this was not simply hard-wired, the researchers from Tel Aviv University grew E. coli for several months with lactose, but without maltose. They found that the bacteria gradually changed their behaviour, so that they no longer bothered to switch on the maltose-digesting system (Nature, DOI&colon; 10.1038/nature08112).

Remarkable though these behaviours are, we have probably only scratched the surface of what single-celled organisms can do. With so many still entirely unknown to science, there must be plenty more surprises in store.